Wine oxidation can occur due to excessive and prolonged contact with an oxygen-containing atmosphere. After significant exposure, oxygen can wreak havoc on a wine’s color and aroma. Of course, not all oxygen contact is detrimental. Controlled oxygen exposure during fermentation can help increase yeast viability and may help to limit reductive wine faults. Wine barrel maturation also benefits from the limited and slow ingress of oxygen, increasing tannin polymerization and mouthfeel.
The problems associated with wine oxidation only occur when too much oxygen enters the winemaking process too quickly. Below are five techniques that winemakers can use to mitigate the negative effects of wine oxidation.
High-quality wines come from high-quality grapes and this is certainly true when we discuss wine oxidation. In growing seasons with low temperatures and high humidity or rainfall, the fungus Botrytis cinerea has an opportunity to thrive. Importantly, Botrytis releases an enzyme called laccase into the grape berry, which ultimately makes its way into the wine. Laccase, in combination with oxygen, will oxidize phenols to quinones, compounds responsible for the destruction of the wine’s desirable aromas.
Laccase is hard to remove once it enters the must, and it survives in higher concentrations of sulfur dioxide. Thus, the best way to prevent wine oxidation due to Botrytis is to control the infection in the vineyard. Good canopy management is key to achieving this and can greatly reduce the amount and spread of the Botrytis infection. Keeping an open canopy that allows airflow can help to reduce moisture on the berries. If Botrytis is present, ensure that oxygen exposure is kept to an absolute minimum as laccase is oxygen requiring. A great source of information for treating laccase is the Australian wine research centre’s fact sheet: “Managing Botrytis infected fruit”.
Important in the wine-making process, is the movement of wine from tank to barrel, to bottle. This process involves the flow of wine into empty (i.e., air-filled) vessels, where contact of wine with oxygen is inevitable. Especially in Botrytis infected fruit, oxygen exposure must be reduced to an absolute minimum.
Using an inert gas cover is great ways to limit oxygen exposure to both must and wine. Options may include using carbon dioxide (CO2), nitrogen, and argon. CO2 is commonly used during the initial grape processing stage. CO2, as dry ice, is placed on the grape must surface where it sublimates and forms a protective blanket. Racking must from the tank to tank can also require the use of inert gas. Post-fermentation, oxygen in the headspace of barrels and tanks is limited by topping the headspace with wine.
Sulfur dioxide (SO2) is a common must and wine additive. Free SO2 (FSO2) rather than bound SO2, helps to reduce wine oxidation and spoilage. FSO2 is comprised of bi-sulfite ions, sulfite ions, and molecular SO2. Molecular SO2 provides the antimicrobial effect, while bi-sulfite and sulfite help to reduce oxidation. At a pH between the range of 2 to 7, FS02 in the form of bisulfite dominates in wine. This form of FSO2, will be the working force behind reducing chemical oxidation.
Sulfite prevents oxidation by inactivating enzymes like laccase, although higher than normal concentrations of sulfite are required to achieve this. The protective capacity of FSO2 is dependent on the wine’s molecular FSO2 concentration, values ranging from 0.6-0.8 mg/L. You can use this handy online calculator, provided by Winebusiness.com, to help you estimate the concentration of molecular FSO2 you require.
Although wine aging in barrel benefits from the slow introduction of oxygen, it is wise to keep exposure to a minimum. Wine barrels being porous in their nature, tend to soak up wine within the staves, increasing the barrel headspace. Ethanol and water in wine can evaporate at the barrel surface, the rate depending on the winery cellar environment. Wine barrels must be topped up with wine frequently to prevent oxygen exposure in the newly formed headspace. Optimizing your barrel stacking arrangements to offer ease in barrel access, can help you to ensure wine topping occurs frequently and efficiently.
Wine aging in bottle can also be a source of wine oxidation. The choice of cork will help to determine how your wine ages in bottle, and how much oxygen will enter overtime. Natural corks are made from Quercus Suber, an evergreen oak tree. Due to its wood origins, it is porous and allows in approximately 0.005-5 mg/L/year into the wine bottle. After several years of aging, the additive effect of oxygen can cause harm if not initially accounted for. Other choices available are synthetic corks or screwcaps, each varying in the level of oxygen ingress they allow into bottle.
Wine oxidation is a key parameter to control for during all stages of winemaking. Utilizing good canopy management practices and optimizing your FSO2 additions are great ways to reduce enzymatic and chemical wine oxidation. Limiting the contact oxygen has with your wine can be achieved by utilizing inert-gas blanketing. Likewise, topping up your barrels frequently, and choosing the right corks, can help prevent the detrimental effects of oxygen.
This article was written by Brittany Goldhawke
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